A steel material used for automobiles, structural materials, and the like is required to be excellent in such mechanical properties as strength, workability, and toughness. In order to improve these properties comprehensively, it is effective to make a steel material with a fine-grained structure; to this end, a number of manufacturing methods to obtain a steel material with a fine-grained structure have been sought. Further, by making the fine-grained structure, it is possible to manufacture a high-strength hot-rolled steel sheet having excellent mechanical properties even if the amount of alloy elements added is reduced.
As a method for making a steel sheet with a fine-grained structure, it is known to carry out a large rolling reduction especially in the subsequent stage of hot finish rolling, deforming austenite grains greatly and increasing a dislocation density; and thereby to obtain fine-grained ferrite after cooling. Further, in view of facilitating the ferrite transformation by inhibiting recrystallization and recovery of the austenite grains, it is effective to cool a steel sheet to 600° C. to 700° C. as quickly as possible after rolling. In other words, subsequent to hot finish rolling, it is effective to rapidly cool a steel sheet after the rolling, by arranging a cooling apparatus capable of cooling more quickly than ever before. And in rapidly cooling a steel sheet after rolling in this way, it is recommended to have a large volume of cooling water sprayed over the steel sheet per unit area, and to have a high flow density in order to enhance a cooling capability.
However, if the volume of water and the flow density are increased in this way, the water accumulated (i.e. retained water) on an upper surface of a steel sheet increases due to a relation between water supply and water discharge. On the other hand, on a lower surface side of a steel sheet, the retained water between a lower surface guide and the steel sheet is likely to increase. This retained water is the water which has remained after being used for cooling the steel sheet. Thus it is desired to discharge the water as quickly as possible, and to provide the steel sheet with water supplied from a cooling nozzle, thereby ensuring a cooling capability. Further, since the retained water is a layer of water, if the layer is thick, the thickness sometimes hinders water from the cooling nozzles from reaching the steel sheet effectively. Furthermore, the retained water flows from a middle portion of the steel sheet toward both ends of the steel sheet in a width direction of the sheet; and the flow rate increases as the water approaches the end portions of the steel sheet. So, if the amount of retained water increases, cooling nonuniformity in the width direction of the steel sheet occurs to a large degree.
Here, looking at the lower surface side of the steel sheet, transporting rolls are arranged on the lower surface side, and a lower surface guide is formed between the transporting rolls to prevent a top portion of the steel sheet from entering therebetween. And this tends to cause the retained water between the lower surface guide and the lower surface of the steel sheet.
Several techniques for suppressing the amount of retained water on a lower surface side of a steel sheet, or for improving a water discharging ability have been disclosed.
For example, Patent Document 1 discloses a configuration in which a slit jet is arranged between transporting rolls, and a space is created over an entire region in a width direction of a cover which covers an entire lower surface of a steel sheet between the transporting rolls in a manner enabling water supply and water discharge by the slit jet.
Patent Document 2 discloses a configuration in which a header for supplying a columnar jet of water and a lower surface guide of a steel sheet are unified, and a cooling nozzle is arranged in the lower surface guide of the steel sheet.
Patent Document 3 discloses a configuration in which a full cone nozzle is used as a cooling means, and a narrow guide is arranged only in the middle of a hot-rolled steel sheet.
Patent Document 4 discloses a configuration in which narrow protection sheets are arranged as a lower surface guide with an appropriate spacing, or a sheet with holes which are combined to form a grid-like shape is used as a lower surface guide, so as to secure discharge of cooling water supplied by a columnar jet.
Patent Document 5 discloses a guide which facilitates discharge of water by arranging a hole to supply cooling water and a hole to discharge the water.